Conservation-tillage cotton brings a change in weed control concerns

If you're planting conservation-tillage cotton in the upper Southeast, and you're burning down winter vegetation, your top five weed problems will be primrose, primrose, primrose, radish and radish.

“In other words, if you take care of these two weeds — primrose and radish — everything else will be taken care of incidentally, says Alan York, North Carolina State University Extension weed specialist. York spoke at the recent Southeast Cotton Conference in Raleigh, N.C.

“We're seeing a significant move towards no-till and strip-till cotton across the Southeast,” says York. “There are a lot of good reasons for it, but the primary reason is reducing the time and expense of so many trips across the field.”

Cutleaf evening primrose, he says, doesn't prefer clay soils. “If conservation-tillage cotton is being grown in the Piedmont, we don't see this weed pest. But in eastern North Carolina, if you till, disk or do something else in the fall, you can bet you'll have primrose in the field, and it'll be growing healthy in the winter,” says York.

It's not known, he adds, how much control of primrose actually is needed in conservation-tillage cotton. “If we're burning down vegetation in the spring, we'll get about 50 percent control. Then, if we're growing Roundup Ready cotton, we'll be putting out glyphosate two more times. So, we'll beat it up pretty good.

“But is that good enough, or do we need to do something more in burndown? We really don't know. We're looking into it, and maybe we'll have a better answer in a year or two. In the short-term, however, we'll continue to be concerned with trying to kill primrose,” he says.

For the past two to three years, says York, the Extension recommendation for primrose control has been one half to one pint of 2,4-D by itself or mixed with something else, applied at least 30 days prior to planting and preferably 45 days before planting, if possible.

“We've found that this recommendation is safe on cotton, and there's no question that it's our most effective option for primrose control. We've looked at cotton lint yields over six tests, with no 2,4-D, one pint and two pints of 2,4-D, applied 28 days prior to planting. We haven't seen a problem. We feel good telling you to apply 2,4-D 30 days ahead of planting. But if you want to go in less than 30 days, you're on your own.”

The effectiveness of 2,4-D and other materials also has been tested, says York. “We looked at the percent of primrose control at 30 to 40 days after application in 11 trials. Glyphosate alone gives us about 60-percent control. A pint of 2,4-D brings us up to 96 percent.”

There's less data on lower rates, he adds, but six trials have been conducted comparing one half and one pint of 2,4-D. “We've seen that reduced rates will work a little slower, but we appear to get the same results after about a month. That would give us an additional margin of safety.”

Many farmers, says York, are hesitant about putting 2,4-D in their cotton sprayers. As a result, questions are being asked about alternatives to 2,4-D, he adds, and many of these materials have been tested against primrose in trials, including Harmony GT, Harmony Extra, Roundup, Goal, Aim, Resource, Clarity and Valor.

Researchers also looked at glyphosate tank-mixes, he says. “Our best material for primrose control still is 2,4-D. Number Two is Clarity, Number Three is Valor and Number Four is Resource. I wouldn't worry about anything below these.”

Clarity, he says, is a brand of dicamba with a label for burndown. The label states that the material must be put out at least 21 days ahead of planting, and that a rainfall event is needed.

“If you can make that 21 days, this material does appear to be safe on cotton, and it works relatively well on primrose. We were getting close to what 2,4-D does for us.”

Valor, says York, is registered for pre-emergence application on peanuts and soybeans and as a burndown on cotton. “The label currently says you need a 30-day waiting period between application and planted cotton. Until we know more about this material, I'd encourage you to stick with the 30 days. Valor would be used at one to two ounces. It looks relatively good, but not as good a 2,4-D.”

Resource has been heavily promoted, he says. “In trials where we used two and three ounces of Resource, we got some help with primrose when we mixed it with glyphosate. But it's still not a good as 2,4-D.”

The North Carolina State Extension recommendation for controlling primrose will continue to be 2,4-D, says York. “We recommend one half to one pint of 2,4-D, preferably in mid-February to mid-March. For those of you who don't want to put 2,4-D in your cotton sprayer, who might want to consider letting someone else put it in their sprayer.”

Wild radish, says the weed scientist, is the other major weed problem for farmers planting conservation-tillage cotton. “Some people call it mustard or other various names, but most of what we see is radish. If, when the plants start blooming in the spring, the flowers have an egg-yellow color, it's probably mustard. But if the flowers are pale yellow to white, it's probably radish.”

Radish, says York, is more difficult to kill. Research from Georgia this past year looked at tank-mixes on wild radish, he says.

“Glyphosate gave 70-percent control, and two thirds ounce of Aim did not help. 2,4-D did look good in these trials. Resource tank-mixed with glyphosate doesn't look very helpful when compared to glyphosate alone. One ounce of Valor did appear to help when compared to glyphosate alone, but it's still second to 2,4-D. Harmony Extra did a good job, and it's about the only tank-mix that gave us control, but it's similar to 2,4-D.”

A big topic of interest during the past couple of years, says York, has been the use of glyphosate/Dual tank-mixes over-the-top of Roundup Ready cotton. Growers in the Southeast, he says, have moved heavily to Roundup Ready cotton, and most have reduced or eliminated their use of soil-applied herbicides.

“We've basically gotten away from residual herbicides in Roundup Ready cotton. Growers now want something to go on postemergence with Roundup to give residual control. Nine times out of 10, they really want something for late-season pigweed, primarily Palmer amaranth. The bulk of these ‘so-called’ late-season weeds are weeds that were missed early, and the farmer didn't see them until later in the season. Nevertheless, many of our growers are convinced that Dual over-the-top will help them.”

The Dual label, says York, states that it can be put over-the-top by itself, on cotton that is three to 12 inches tall. If it's tank-mixed with glyphosate, it must be a directed spray, he says.

“It's my understanding that there will be a label change for this year that will allow glyphosate/Dual tank-mixes over-the-top. We took a hard look at this, with more than 40 trials between North Carolina and Georgia. We feel comfortable saying that the crop tolerance is fine. You'll see some burn on the cotton plant, but it's confined to the leaves that come into contact with the spray solution.

“We haven't seen a problem with stunting, nor have we seen adverse effects on yield or quality. The exception might be if you start pouring a bunch of adjuvants in the tank. If you're going to use a glyphosate/Dual tank-mix, use only a glyphosate material that already contains enough adjuvants.”

“We see residual control up until the layby application. From that point on, we've seen limited benefits. In our research, we've seen limited benefits from a Dual application. What are we doing differently? Obviously, in research, we can do a more timely job with our layby application. We're able to get small weeds with a directed spray, and we get a good kill. Many farmers start their layby on time, but the applications might not be timely on the second half of their acreage.”

Research has shown more benefits from Dual when the application is delayed, says York. “When the application is delayed, we could get a good benefit. But we don't see much of an effect from a timely application.”